If you are talking about Defog(HM05), then you will have to beat Crasher Wake first in order to use it outside of battle. It is somewhere in the Great Marsh.
In Platinum, it's Fantina, and after you beat her, you can use Surf.
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The move Rock Tomb is a TM, not a HM.
Go to the move deleter in Canalave City
you can use the hm strength you can smash rocks with rock smash
at canalave city there is a move deleter he is an old man
youdon't defog is a hm move that is in Pokemon diamond, pearl, and platinum